Thermal radiation, temperature and energy

[EHLO]

One for a patient physicist - coming from an electronics background I basically understand the emission of EM radiation/photons by oscillating charges, electron transition through a bandgap, nuclear decay and spontaneous emission, but can't seem to recall a conceptual mechanism for thermal, or black body, radiation.

We have Planck's Law, that describes the black body spectrum, but what's the source of the photon emission?

Another question that is related and may highlight my ignorance in the matter, or at least give you an idea of the mental picture I have; Can a single molecule, say a hydrogen gas molecule, in a vacuum have a "Temperature", and if so does it emit BB radiation?

 

[Walter Wayne]

Not a physicist, but as I recall in addition the transition of electrons from higher states to lower, you also have other energy states to consider in your basic molecule. Two bound atoms will vibrate to and from each other, in a "spring" type motion, and they can also spin around each other. The vibration of the atoms is quantized in similar manner to electrons (except that multiple atoms can have the same vibrational state unlike electrons).

When a material is above zero temperature, atoms will be vibrating more fall into a lower vibrational state. It is that drop in kinetic energy of the atoms that emit photons.

So when you turn on an incandescent light bulb the electricity passing through heats up the filament, shaking the hell out of the atoms, and this means some of the vibration is at high enough energy to emit in the visible spectrum.

Some physicist will probably come by with a better explaination.

Walt

 

[EHLO]

Thanks for that. I thought of intra-molecular bonds initially, but it doesn't make sense for something like a monatomic gas, say Helium.

I've been hunting around for info but am either missing something obvious or looking in the wrong place.

Temperature is a measure of average kinetic energy, so presumably thermal radiation is a consequence of inter atomic/molecular interactions. Hence the query as to whether a single atom/molecule has temperature, or emits BB radiation. If not, how many do you need?

 

[ingoa]

In physics, a black body is an object that absorbs all electromagnetic radiation that falls onto it. No radiation passes through it and none is reflected. It is this lack of both transmission and reflection to which the name refers. These properties make black bodies ideal sources of thermal radiation. That is, the amount and spectrum of electromagnetic radiation they emit is directly related to their temperature. Black bodies below around 700 K (430 °C) produce very little radiation at visible wavelengths and appear black. Black bodies above this temperature however, produce radiation at visible wavelengths starting at red, going through orange, yellow, and white before ending up at blue as the temperature increases.
The term "black body" was introduced by Gustav Kirchhoff in 1860. The light emitted by a black body is called black-body radiation (or cavity radiation), and has a special place in the history of quantum mechanics.[1]

fom http://en.wikipedia.org/wiki/Black_body


As a single atom only absorbs radiation of certain wavelengths it cannot be a black body. This means also that it can only radiate in the same wavelengths. Hence, no BB radiation.

You will need a collection of atoms so that ALL wavelengths will be absorbed eventually. This may go via scattering, excitation of molecules and so forth. At the end it will be what physicists call heat. So you will need many atoms. I would assume that the minimum mass would be in the order of grams or at least milligrams, for sure not nanograms.

This body, gas or liquid could then also emit radiation of all wavelengths, which make thermal radiation possible and voilá you arrive at Planck's spectrum (some practical details omitted).

 

[sol invictus]

One for a patient physicist - coming from an electronics background I basically understand the emission of EM radiation/photons by oscillating charges, electron transition through a bandgap, nuclear decay and spontaneous emission, but can't seem to recall a conceptual mechanism for thermal, or black body, radiation.

All of the above. Thermal radiation is produced by any process within the system which is capable of emitting photons.

Can a single molecule, say a hydrogen gas molecule, in a vacuum have a "Temperature", and if so does it emit BB radiation?

In an absolute vacuum, no. A single hydrogen will be in some definite state; at best it can emit a few photons of certain definite frequencies. You couldn't really call that thermal, since you wouldn't have a spectrum. After the emission it will have decayed to its ground state, and that's the end of the story.

A gas of such molecules could be thermal. How many you need for that doesn't have a definite answer - the more you have, the more precise the thermodynamic description gets. Remember, there are something like 10^22 molecules per liter of gas.

 

[EHLO]

Thanks all, I gather from these responses that perhaps my interpretation of a "black body" was off, in that this is an ideal case where emission is a continuous probability distribution.

For the case of, say, a body consisting of only Hydrogen gas, the thermal radiation would be a series of peaks corresponding to the various modes of emission, scaled by the ideal BB distribution. (if that makes sense)